The disclosed subject matter relates generally to semiconductor device manufacturing and, more particularly, to a method and apparatus for performing double exposure photolithography using a single reticle.
Semiconductor devices, or microchips, are manufactured from wafers of a substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the integrated, electrical circuits that make up the device. The fabrication essentially comprises four operations: layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced; patterning, or removing selected portions of added layers; doping, or placing specific amounts of dopants in the wafer surface through openings in the added layers; and heat treatment, or heating and cooling the materials to produce desired effects in the processed wafer. Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process.
The fabrication process generally involves processing a number of wafers through a series of fabrication tools. Each fabrication tool performs one or more of the four basic operations. The four basic operations are performed in accordance with an overall process to finally produce wafers from which the semiconductor devices are obtained.
Of these four operations, patterning is considered to be an important step. Patterning is known to those in the art by many names. Other names for patterning include photolithography, photomasking, masking, oxide removal, metal removal, and microlithography. The term “photolithography” will hereafter be used to refer to patterning operations. Photolithography typically involves a machine called an “exposure tool,” or sometimes also called a “stepper” or a “scanner”. An exposure tool positions a portion of a wafer being processed under a “photomask.” The photomask is usually a reticle, which is a copy of a pattern created in a layer of chrome on a glass plate. Light is then transmitted through the reticle onto a thin layer of material called photoresist previously added to the wafer. The chrome blocks the light while the glass allows it to pass.
The light shining through the pattern on the reticle creates an aerial image which, when interfacing with the photoresist at the optimum focal plane, changes the material characteristics of the photoresist where it shines. In essence, this allows the pattern on the reticle to be duplicated in, or transferred to, the photoresist. The change in material characteristics makes the photoresist susceptible to removal in the subsequent develop operation prior to the next sequential process step such as etching or ion implantation. The exposure tool then positions another portion of the wafer under the reticle, and the pattern transfer is repeated. The process is repeated until the entire wafer has completed the pattern transfer operation. This process of shining light through a photomask to treat a photoresist is known as “exposure,” or “pattern transfer.”
The reticle described in the example above is more precisely known as a “binary mask” because each portion of the reticle either transmits all the light or blocks all the light. However, ever-decreasing feature sizes have created problems for binary masks. The light shining through the chrome pattern scatters at the edges of the chrome traces, with undesirable effects on the pattern transfer process to the photoresist. The smaller the feature sizes, the more acute the problem.
Another type of photomask is a “phase shift” photomask. There are a variety of phase shift photomask types, but all shift the phase of the light waves so that the projected image of the photomask has an improvement of one or more image characteristics (e.g., contrast, edge definition, etc.) as compared with the same pattern from a binary photomask. An attenuated phase shift photomask, for instance, comprises a reticle that attenuates and phase-shifts the light wave in the “dark” regions of the photomask so that the contrast between bright and dark regions of the image is improved. Since, the transmission function of such a photomask cannot be described in simple terms of “bright” or “dark,” this type of mask is not considered “binary.” A complementary phase shift photomask actually comprises two reticles, where, at most, only one of which can be binary. The first (i.e., typically binary) is used to define an exposure area and to expose noncritical features, and the second (i.e., typically phase-shifting) is used to expose the critical features in a second pass. Both passes are performed before the wafer is stepped to process another portion of the wafer so that the wafers are not exposed, developed, baked, and etched twice.
Optical lithography systems all share a fundamental physical limitation on the minimum pitch (i.e., the center-to-center space of two adjacent features) that can be resolved. This limit is a function of the illumination wavelength and the numerical aperture (NA) of the exposure tool.
One technique for etching features is referred to as spacer double patterning. A template is formed on the layer, and spacers are formed around the template. The template is removed and the material not covered by the spacers is etched away to form the underlying pattern. One limitation of spacer double patterning is that the critical dimension (CD) of the patterned feature is defined by the spacer width. The spacers are formed by depositing a spacer layer of uniform thickness across the wafer. The spacer layer is then etched to form the spacers, where the CD of the spacers is defined by the thickness of the spacer layer. Since the spacer film thickness defines the CD of the circuitry and the spacer film is uniformly deposited across the wafer, it is not feasible to create features having different CDs without using an additional mask for each unique CD. This need for additional masks and the resulting additional photolithography steps makes the use of such a technique not cost-effective.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.